commit 4a31ffa2f0097d96201f80305a0495c57552f0ad
author Neels Hofmeyr <neels@hofmeyr.de> Thu Sep 07 03:08:06 2017 +0200
committer Neels Hofmeyr <nhofmeyr@sysmocom.de> Tue Sep 19 01:35:30 2017 +0000
tree 65e45b9fcbb5782aa7e50e788c46798f934c76a5
parent a52d839343aea9b81c2463f52c3c3358778698f3

VTY: implicit node exit by de-indenting, not parent lookup

Note: This will break users' config files if they do not use consistent
indenting. (see below for a definition of "consistent".)

When reading VTY commands from a file, use indenting as means to implicitly
exit child nodes. Do not look for commands in the parent node implicitly.

The VTY so far implies 'exit' commands if a VTY line cannot be parsed on the
current node, but succeeds on the parent node. That is the mechanism by which
our VTY config files do not need 'exit' at the end of each child node.

We've hit problems with this in the following scenarios, which will show
improved user experience after this patch:

*) When both a parent and its child node have commands with identical names:

  cs7 instace 0
   point-code 1.2.3
   sccp-address osmo-msc
    point-code 0.0.1

If I put the parent's command below the child, it is still interpreted in the
context of the child node:

  cs7 instace 0
   sccp-address osmo-msc
    point-code 0.0.1
   point-code 1.2.3

Though the indenting lets me assume I am setting the cs7 instance's global PC
to 1.2.3, I'm actually overwriting osmo-msc's PC with 1.2.3 and discarding the
0.0.1.

*) When a software change moves a VTY command from a child to a parent. Say
'timezone' moved from 'bts' to 'network' level:

  network
   timezone 1 2

Say a user still has an old config file with 'timezone' on the child level:

  network
   bts 0
    timezone 1 2
    trx 0

The user would expect an error message that 'timezone' is invalid on the 'bts'
level. Instead, the VTY finds the parent node's 'timezone', steps out of 'bts'
to the 'network' level, and instead says that the 'trx' command does not exist.

Format:

Consistent means that two adjacent indenting lines have the exact
same indenting characters for the common length:

Weird mix if you ask me, but correct and consistent:

  ROOT
  <space>PARENT
  <space><tab><space>CHILD
  <space><tab><space><tab><tab>GRANDCHILD
  <space><tab><space><tab><tab>GRANDCHILD2
  <space>SIBLING

Inconsistent:

  ROOT
  <space>PARENT
  <tab><space>CHILD
  <space><space><tab>GRANDCHILD
  <space><tab><tab>GRANDCHILD2
  <tab>SIBLING

Also, when going back to a parent level, the exact same indenting must be used
as before in that node:

Incorrect:

  ROOT
  <tab>PARENT
  <tab><tab><tab>CHILD
  <tab><tab>SIBLING

As not really intended side effect, it is also permitted to indent the entire
file starting from the root level. We could guard against it but there's no
harm:

Correct and consistent:

  <tab>ROOT
  <tab><tab>PARENT
  <tab><tab><tab><tab>CHILD
  <tab><tab>SIBLING

Implementation:

Track parent nodes state: whenever a command enters a child node, push a parent
node onto an llist to remember the exact indentation characters used for that
level.

As soon as the first line on a child node is parsed, remember this new
indentation (which must have a longer strlen() than its parent level) to apply
to all remaining child siblings and grandchildren.

If the amount of spaces that indent a following VTY command are less than this
expected indentation, call vty_go_parent() until it matches up.

At any level, if the common length of indentation characters mismatch, abort
parsing in error.

Transitions to child node are spread across VTY implementations and are hard to
change. But transitions to the parent node are all handled by vty_go_parent().
By popping a parent from the list of parents in vty_go_parent(), we can also
detect that a command has changed the node without changing the parent, hence
it must have stepped into a child node, and we can push a parent frame.

The behavior on the interactive telnet VTY remains unchanged.

Change-Id: I24cbb3f6de111f2d31110c3c484c066f1153aac9